Computed radiography uses storage phosphors to record the radiation image from x-ray transmission. The stored image is then read out by light-stimulated emission, which is amplified by a photomultiplier, transformed into desired signal units (such as log-exposure) and quantified into digital numbers called code values. The code values can be, recorded with a certain number of bits. For a 12-bit digital image, the code value ranges from 0 to 4095. These code values are then manipulated by computers and then mapped through a tone-scale curve to an output display, such as a cathode-ray-tube monitor or a photographic film.
Computed radiography using storage phosphors offers a very wide exposure latitude (10,000:1) compared with that of the conventional screen/film systems, (40:1). This means that exposure error is much less serious for computed radiography at the time of image sensing and recording (but not at the time of image display, as will be explained later). It also means that the tone scale mapping in computed radiography can be specifically tailored to provide an optimal rendition of every individual image. However, most output media, such as photographic film and cathode ray tube (CRT), do not have wide enough dynamic ranges to display the 10,000:1 latitude of information with proper visual contrast. It is, therefore, necessary to carefully allocate the available output dynamic range to display the clinically important part of the input code values. For some applications, the interested range of the input image may exceed that provided by the output media and the contrast of parts of the input image will have to be compromised. U.S. Pat. No. 4,302,672 teaches a method of constructing such a compromised tone-scale curve for chest x-ray images. However, that method uses the valleys and peaks of the code-value histogram to identify the critical points between the spine, the heart, and the lung. The results are not very reliable because these valleys and peaks are not always clearly detectable. Furthermore, the method cannot be generalized to examinations other than chest images.
There are mainly five classes of "objects" in radiographic images: (1) the foreground (collimator blades used to protect parts of the body from unnecessary x-ray exposure) usually corresponding to very low to low exposure areas; (2) the man-made objects (such as pacemakers, tubes, and electrodes); (3) the soft tissues (such as muscles, blood vessels, and intestines) usually correspond to low (e.g., mediastinum) to high (e.g., lung) exposures depending on the thickness; (4) the bones corresponding to the very low to low exposures (often overlaps with the foreground); and (5) the background corresponding to the very high exposure areas. These five classes of objects are very difficult to separate using the code value alone because, there are considerable overlaps (such as the bone and the collimator blades).
The two basic problems in the tone scale adjustment for computed radiography are: (1) to determine which sub-range of the input code values is most important for clinical evaluation and (2) to construct a tone-scale transfer curve so that the important sub-range of the code values identified in step (1) can be rendered with proper contrast and brightness (density) on the output media. For example, the digital code values of an input chest x-ray image may span from 500 to 3000 (in units of 0.001 log exposure), but, the code value range of the lung area, being the most important region of the image, may only span from 1800 to 2600. If we attempt to map the entire range of the input code value (from 500 to 3000) to the available film density range with equal contrast for all input code values, we will produce a chest image with an unacceptably low contrast. It is therefore very important to have an automatic algorithm to detect and select the relevant sub-range of the input code values (1800 to 2600) to be displayed on the output media with proper visual contrast and brightness. The process of selecting the relevant sub-range of input code values and constructing the proper mapping function from the input code value to the output display media is called the tone scale adjustment.
The first problem, selecting the important code value sub-range, has been approached in the past by using the histogram of the input code values either in exposure space or in log-exposure space. (We will refer this histogram as the code-value histogram to differentiate it from the image activity histogram used in this invention.) U.S. Pat. No. 4,914,295 teaches a method for excluding the irrelevant background region in an image by detecting a peak in the code-value histogram near the maximum input code value. This method requires a prior filter to remove the image region covered by the collimator blades. It also needs a different preconstructed tone-scale curve in accordance with the examination type entered by the operator. U.S. Pat. No. 5,046,118 discloses a method of analyzing the code-value histogram by an entropy measure. U.S. Pat. No. 5,268,967 teaches a method of using edge density for segmenting image into foreground, background, and body part regions. It also relies on the prior knowledge of the body part being examined.
All the prior art methods attempted to perform the tone scale adjustment of an input image based on the analysis of its code-value histogram. A code-value histogram contains only the information about the number of pixels at various exposure or log-exposure values. It does not tell us which code value sub-range, contains the important body part image. Analysis of this type of histogram is thus always performed under various ad hoc assumptions about the image contents. When the assumptions are not valid for an input image, these methods tend to produce a very unacceptable output image.
The second problem, constructing the proper tone-scale curve, has been approached in the past in two different ways. U.S. Pat. No. 4,641,267 teaches a method of providing a set of reference tone-scale curves (about 10 of them) to choose from and for each chosen reference curve a slight adjustment of exposure and contrast is made based on the code-value histogram. The tone-scale curve used depends only slightly on an individual image, but rather mostly on its examination type. This has the advantage of providing a consistent appearance of the final displayed images for any given examination type. U.S. Pat. No. 5,164,993 teaches another method for constructing a tone-scale transfer curve based on the code-value histogram, such that the output density is substantially a linear function of the input exposure. The resulting tone-scale curve is highly image dependent. This has the potential of providing a customized, optimal tone-scale curve for each individual image. However, both methods rely on the code-value histogram and do not consider the characteristics of human visual system for tone-scale construction, Both methods also require separate preprocessing procedures for eliminating the image regions that correspond to the background and the collimator blades.
A good tone-scale curve should render the detailed structure of the input image easily visible. Image regions that contain modulated signals should be allocated with sufficient contrast and image regions that show little signal modulation can be rendered with low contrast. Signal modulations represent image activities and should be measured as a function of code values.